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sol_ther_lecture2.pptx
1. Heat Transfer Concepts: Theory of
thermal radiation, Radiation properties,
Planck’s law, Wien’s displacement law,
Stefan-Boltzmann equation; Concept of
black body and gray body; Spectral
dependence of radiation properties;
Kirchhoff’s law; Shape factor; Radiation
exchange between surfaces, Re-
radiating surfaces; Radiation shields.
2. 2
INTRODUCTION Radiation differs from conduction and
convection in that it does not require the
presence of a material medium to take place.
Radiation transfer occurs in solids as well as
liquids and gases.
The hot object in vacuum
chamber will eventually cool
down and reach thermal
equilibrium with its
surroundings by a heat transfer
mechanism: radiation.
3. 3
Accelerated charges or changing electric currents give rise to electric and
magnetic fields. These rapidly moving fields are called electromagnetic waves or
electromagnetic radiation, and they represent the energy emitted by matter as a
result of the changes in the electronic configurations of the atoms or molecules.
Electromagnetic waves transport energy just like other waves and they are
characterized by their frequency or wavelength . These two properties in a
medium are related by
c = c0 /n
c, the speed of propagation of a wave in that medium
c0 = 2.9979108 m/s, the speed of light in a vacuum
n, the index of refraction of that medium
n =1 for air and most gases, n = 1.5 for glass, and n = 1.33 for water
It has proven useful to view electromagnetic radiation as the propagation
of a collection of discrete packets of energy called photons or quanta.
In this view, each photon of frequency n is considered to have an energy of
The energy of a photon is inversely
proportional to its wavelength.
4. 4
THERMAL RADIATION
The
electromagnetic
wave spectrum.
The type of electromagnetic radiation that is pertinent to heat transfer is the
thermal radiation emitted as a result of energy transitions of molecules, atoms,
and electrons of a substance.
Thermal radiation is that electromagnetic radiation emitted by a body as a result
of its temperature
Temperature is a measure of the strength of these activities at the microscopic
level, and the rate of thermal radiation emission increases with increasing
temperature.
Thermal radiation is continuously emitted by all matter whose temperature is
above absolute zero.
Everything
around us
constantly
emits thermal
radiation.
6. 6
Light is simply the visible portion of
the electromagnetic spectrum that
lies between 0.40 and 0.76 m.
A body that emits some radiation in the
visible range is called a light source.
The sun is our primary light source.
The electromagnetic radiation emitted by
the sun is known as solar radiation, and
nearly all of it falls into the wavelength
band 0.3–3 m.
Almost half of solar radiation is light (i.e.,
it falls into the visible range), with the
remaining being ultraviolet and infrared.
The radiation emitted by bodies at room temperature falls into the
infrared region of the spectrum, which extends from 0.76 to 100 m.
The ultraviolet radiation includes the low-wavelength end of the thermal
radiation spectrum and lies between the wavelengths 0.01 and 0.40 m.
Ultraviolet rays are to be avoided since they can kill microorganisms and
cause serious damage to humans and other living beings.
About 12 percent of solar radiation is in the ultraviolet range. The ozone
(O3) layer in the atmosphere acts as a protective blanket and absorbs
most of this ultraviolet radiation.
7. 7
In heat transfer studies, we are interested in
the energy emitted by bodies because of their
temperature only. Therefore, we limit our
consideration to thermal radiation.
The electrons, atoms, and molecules of
all solids, liquids, and gases above
absolute zero temperature are constantly
in motion, and thus radiation is
constantly emitted, as well as being
absorbed or transmitted throughout the
entire volume of matter.
That is, radiation is a volumetric
phenomenon.
8. 8
BLACKBODY RADIATION
• A blackbody is a perfect emitter and absorber of radiation.
Stefan–Boltzmann constant
Blackbody emissive power
The radiation energy
emitted by a blackbody:
A body s considered to absorb all incident radiation from all directions at
all wavelengths without reflecting, transmitting or scattering it. For a given
temperature and wavelength, no other body at the same temperature can
emit more radiation than a blackbody. The radiation emission by a black
body at any temperature T is the maximum possible emission at that
temperature
9. 9
The fundamental quantity that specifies the magnitude of the radiation
energy emitted by a black body at an absolute temperature T, at a
wavelength in any given direction is called spectral black body
radiation intensity Ib(T)
Spectral black body radiation intensity
Plank’s law describes the amount of energy emitted by a black body in
radiation of a certain wavelength (i.e. spectral radiance of a black body).
The law is named after Max Plank, who originally proposed it in 1900.
13. 13
Black body Emissive Power
The spectral radiation energy emitted by the surface element dA,
streaming through an elemental solid angle d in any given direction
15. 15
Spectral blackbody emissive Power:
Emissive power is defined as the energy
radiated from a body per unit area per
unit time at an absolute temperature T.
Boltzmann’s constant
Planck’s
law
The amount of radiation energy emitted
by a blackbody at a thermodynamic
temperature T per unit time, per unit
surface area, and per unit wavelength
about the wavelength .
16. 16
The wavelength at which the
peak occurs for a specified
temperature is given by
Wien’s displacement law:
19. 19
Radiation from real surfaces:
The spectral radiation intensity emitted by a real surface at temperature T of
wavelength is always less than that emitted by a black body at the same
temperature and wavelength. Furthermore radiation intensity from a real
surface depends on direction, whereas the black body radiation intensity is
independent of direction.
24. 24
RADIATIVE PROPERTIES
Most materials encountered in practice, such as metals, wood, and bricks, are opaque
to thermal radiation, and radiation is considered to be a surface phenomenon for such
materials.
Radiation through semitransparent materials such as glass and water cannot be
considered to be a surface phenomenon since the entire volume of the material
interacts with radiation.
A blackbody can serve as a convenient reference in describing the emission and
absorption characteristics of real surfaces.
Emissivity
• Emissivity: The ratio of the radiation emitted by the surface at a given temperature
to the radiation emitted by a blackbody at the same temperature. 0 1.
• Emissivity is a measure of how closely a surface approximates a blackbody ( = 1).
• The emissivity of a real surface varies with the temperature of the surface as well as
the wavelength and the direction of the emitted radiation.
• The emissivity of a surface at a specified wavelength is called spectral emissivity
. The emissivity in a specified direction is called directional emissivity where
is the angle between the direction of radiation and the normal of the surface.
26. 26
A surface is said to be diffuse if its properties are independent of
direction, and gray (Gray body) if its properties are independent of
wavelength.
The gray and diffuse approximations are often utilized in radiation
calculations.
30. 30
In practice, surfaces are assumed to reflect in a perfectly specular or diffuse manner.
Specular (or mirrorlike) reflection: The angle of reflection equals the angle of
incidence of the radiation beam.
Diffuse reflection: Radiation is reflected equally in all directions.
31. 31
Kirchhoff’s Law
The total hemispherical emissivity of
a surface at temperature T is equal
to its total hemispherical absorptivity
for radiation coming from a
blackbody at the same temperature.
Kirchhoff’s law
spectral form of
Kirchhoff’s law
The emissivity of a surface at a specified wavelength, direction, and
temperature is always equal to its absorptivity at the same wavelength,
direction, and temperature.
32. 32
The Greenhouse Effect
Glass has a transparent window in the wavelength range 0.3 m < < 3 m in which
over 90% of solar radiation is emitted. The entire radiation emitted by surfaces at room
temperature falls in the infrared region ( > 3 m).
Glass allows the solar radiation to enter but does not allow the infrared radiation from the
interior surfaces to escape. This causes a rise in the interior temperature as a result of
the energy buildup in the car.
This heating effect, which is due to the nongray characteristic of glass (or clear plastics),
is known as the greenhouse effect.
33. The value of the total solar irradiance can be used
to estimate the effective surface temperature of
the sun from the requirement that
The sun can be treated
as a blackbody at a
temperature of 5780 K.
33
35. it is found convenient in radiation calculations to treat the atmosphere as a
blackbody at some lower fictitious temperature that emits an equivalent amount
of radiation energy.
This fictitious temperature is called the effective sky temperature Tsky.
The radiation emission from the atmosphere to the earth’s surface is
The value of Tsky depends on the atmospheric
conditions. It ranges from about 230 K for cold,
clear-sky conditions to about 285 K for warm,
cloudy-sky conditions.
Net rate of radiation heat transfer to a surface
exposed to solar and atmospheric radiation
35
